This invention relates to an engine control system and more particularly to an improved feedback control system for internal combustion engines.
In the interests of improving fuel economy and exhaust emission control, a wide variety of types of engine control systems have been proposed. Many forms of these systems employ combustion conditions sensors such as oxygen sensors. These sensors are capable of determining the actual air-fuel ratio in the combustion chamber at the time of combustion. An oxygen sensor (O.sub.2) operates to do this by measuring the amount of oxygen remaining in the residual exhaust gasses. From this, it is possible to obtain an indication whether the air-fuel ratio is at the desired value.
By utilizing such a control, it is possible to actually adjust the engine's running conditions from cycle to cycle and achieve good exhaust emission control and good fuel economy. These types of sensors, and particularly the oxygen type sensor, however provide generally an on/off signal. That is, the sensor basically changes its state from a rich signal to a lean signal when deviations from stoichiometric occur. These sensors, however, since they are exposed directly to the exhaust gasses can become contaminated and fail.
Failure, of course, is relatively easy to detect and arrangements can be made for other types of controls upon such failure. However, at times the sensor may not have a complete failure, but may output a signal that is not truly representative of the air-fuel ratio. If this information is utilized for engine control, then poor control may result.
It is, therefore, a principal object of this invention to provide an improved engine control system embodying a feedback control and sensor and which will provide for compensation for the control in the event the sensor output degrades.